1. Field of the Invention
This invention relates to implantable cardiac pacemakers of the kind which are externally programmable for changing certain operating parameters. More particularly, the invention relates to a method of externally determining the actual parameter values which have been programmed into an implanted pacemaker.
2. The Prior Art
Programmable pacemakers have provisions for the external programming of various parameters, such as stimulation pulse rate, escape interval, refractory period, pulse width, pulse amplitude, and sensitivity. Also programmable is the mode of operation, such as demand, synchronous and asynchronous.
Communication between the external programmer and the pacemaker may be by means of radio waves, ultrasound, by magnetic fields, or by other means. Usually, the pacemaker is provided with a reed switch which may be closed by the application of an external magnet, thereby causing the pacemaker to be switched into asynchronous mode. A programmable pacemaker of this kind is described in U.S. Pat. No. 4,124,031. According to this patent, the magent is also used for programming, in that it is alternately removed from and applied to the patient in predetermined sequences of varying durations. Although this method of programming has the advantage of not requiring any special programming apparatus, it has the disadvantage that the pacemaker may be inadvertently programmed incorrectly if a mistake is made in the sequence of applications and removals of the magnet. It is therefore desirable, in a programmable pacemaker, to have the possibility of checking the actual programmed settings. It should be noted that apart from the stimulation pulse rate, most of the programmable parameters mentioned above are not observable on ECG equipment.
A circuit for determining the parameter control states of an implanted pacemaker is described in U.S. Pat. No. 4,190,055. According to this patent, the contents of a binary register containing the control inputs to the parameter controlling circuitry of the pacemaker are converted into serial form by means of a parallel to serial converter. The appearance of one logic state in the serial output causes a delay to be added between simulating pulses, while the appearance of the other logic state does not affect the pulse rate, so that for instance a logic one will appear as a longer interval between pulses than the intervals representing a logic zero.